The present invention relates to improvements in a method of manufacturing a glass passivation type semiconductor device.
In recent years, in semiconductor elements for medium and low power levels, the glass passivation type in which recesses reaching a p-n junction are provided in a silicon wafer to expose the p-n junction in the recesses and in which the exposed parts of the p-n junction are covered with a low-melting glass has become the mainstream owing to ease in the mass processing of surface protection.
A prior-art device of this type will be described with reference to FIGS. 5 to 7. In the figures, numeral 1 designates an n-type silicon substrate, which is formed by well-known diffusion techniques with a p-type diffused layer 2 and an n.sup.+ diffused layer 3 for attaining an ohmic contact. Numeral 4 designates exposed portions where silicon parts are exposed, numeral 5 covered portions which are covered with an SiO.sub.2 film, a photoresist film or the like capable of withstanding a silicon etchant of a nitric acid--fluoric acid system, numeral 6 a p-n junction, numeral 7 a recess the p-n junction 6 is exposed, numeral 8 a low-melting passivation glass which is buried in the recess 7 and which is vitrified by treatment, and numerals 9 and 10 electrode portions which are respectively formed by a process such as vacuum deposition or plating.
In the device constructed as described above, the p-type diffused layer 2 and the n diffused layer 3 are first formed in the n-type silicon substrate 1 by well-known diffusion techniques, whereupon the exposed portions 4, where the silicon parts are exposed, and the covered portions 5, which are covered with the SiO.sub.2 film, the photoresist film or the like, are formed by photoengraving. Subsequently, the exposed portions 4 are etched to expose the p-n junction 6 in the recesses 7. Thereafter, while care is taken of the degree of cleanness of the exposed parts of the p-n junction 6, the low-melting passivation glass 8 is buried in the recesses 7 and is vitrified by a heat treatment. Thereafter, the electrode portions 9 and 10 are formed by vacuum evaporation, plating, etc. as shown in FIG. 6, and positions indicated by dot-and-dash lines in the figure are cut to obtain a plurality of diodes.
In case of burying the passivation glass 8 in the recesses 7, a printing method and a doctor blade method are more effective than electrodeposition and sedimentation in consideration of mass-producibility, and hence, any of the methods is employed. The passivation glass 8 is subsequently heat-treated under predetermined conditions, whereby it is secured in the recesses 7. Next, a method of depositing the glass will be described in detail. The electrodeposition and the sedimentation are difficult of controlling the thickness of a glass film and cause very great deviations in the film thickness, and they are also inferior in the processing ability. On the other hand, the printing method and the doctor blade method are favorable in the mass producibility, but they sometimes deposit the glass imperfectly or deposit the glass to unnecessary parts. This will be explained with reference to FIG. 7. 11 indicates undeposited portions where the passivation glass 8 is not deposited, and 12 unnecessary portions where the passivation glass 8 is deposited. That is, even with the printing method and the doctor blade method, the undeposited portions 11 and the unnecessary portions 12 arise.
As stated above, with the prior-art method, the passivation glass is deposited to only the recesses 7. In order to prevent the passivation glass from depositing around the recesses 7, an emulsion is applied between a mask and the silicon substrate. However, a glass paste with the glass and a solvent mixed can ooze out to the side of the emulsion, resulting in the problem that the glass is deposited on unnecessary portions.